Add Support for Swift Package Manager (Minor Update)

[robfeldmann]

This PR includes the previous pull request from @Tkhp and adds the following minor modifications:

• adds the other supported platforms (tvOS and watchOS) to the Package.swift file (these are listed as being supporting in the .podspec)
• adds .swiftpm to the .gitignore file (pretty sure these should be ignored)

As a refresher, @Tkhp submitted #33:

As of Xcode 11, Xcode has Swift packages integration so we can now install R.swift.Library in Xcode by Swift Package Manager.
I wrote a basic Package.swift and a manual to install R.swift.Library with Xcode's Swift packages integration.
I've tested it and migrated from Carthage in my private projects, and everything works well.

I can also confirm this works great. Hopefully these will be accepted soon.

Thanks!

[tomlokhorst]

Thanks for this update!

Have you by any change gotten R.swift to work with SPM on watchOS?
I can't get it to import on watchOS, but it does work with tvOS, so I'm assuming it's some Xcode beta thing.

[kylebrowning]

@tomlokhorst Thanks for merging this, but FYI 5.1.0.alpha.1 is not a valid semantic version, and cannot be resolved by SPM.

[kylebrowning]

And the project currently doesn't resolve on latest 5.0.1 because it does not have Package.swift in it.